Ceramic materials generally have excellent hardness, heat resistance, abrasion resistance, and corrosion resistance, and are therefore desirable for high temperature applications such as in gas turbine engines and the like. However, ceramic materials are easily fractured by tensile stresses and exhibit a high degree of brittleness. To improve upon the fracture toughness of a ceramic material, it is known to provide a ceramic matrix composite (CMC) material wherein inorganic fibers are disposed in a matrix of ceramic material. The fibers provide tensile strength and toughness to augment the other desirable properties of the ceramic material. A CMC material may be formed by impregnating a preform of ceramic fiber-containing fabric material with a ceramic precursor material, such as by a known wet method such as slip casting or slurry infiltration, or by decomposing a chemical compound in the vapor form and depositing the resulting ceramic onto the fiber preform, or by using a sol-gel method or using a polymer pre-cursor material. The cast or laid-up part is then compacted and dried using low or high pressures and temperatures to form a green body. The term “green body” is used to denote a dried ceramic form or otherwise weakly bonded ceramic powder compact material prior to being fired, including such a material when used as a matrix material surrounding fibers in any state. The firing process drives off additional water and organic material and converts the green material to a denser, stronger more refractory ceramic material. Firing may be accomplished by known techniques such as atmospheric pressure sintering or reaction sintering which sinter the matrix to its final density to form the refractory ceramic matrix composite material.
One example of a commercially available oxide fiber/oxide matrix CMC material is an alumina-silica fiber/alumina matrix composite available from COI Ceramics, Inc. of San Diego, Calif. U.S. Pat. No. 4,047,965, incorporated by reference herein, describes a method of manufacturing such fibers that includes the steps of firing green fibers to remove water and organic material to form the final refractory fibers. It is known in the art to form CMC materials by surrounding fully fired (stabilized) ceramic fibers with a ceramic matrix material, and then drying and firing the composite at a temperature that is sufficiently high to sinter the matrix material without being high enough to damage the already-stabilized fibers.